Two angles are coterminal if they differentiate in a factor of k*(2pi). This is a multiple of 360 degrees.
In this case 21 [pi /10] = [pi / 10] + 20 [pi/10] = [pi/10] + 2pi, then 21pi/10 is coterminal with pi/10.
In the case of -pi/10 you cannot get to pi/10 by adding a multiple of 2pi , then they are not coternminals.
Conclusion: you would not be right.
